Invading U.S. troops were first seen from this now-abandoned tower, leading to the Spanish-American War.

This nearly abandoned "company town" is a ghost of Puerto Rico's once-lucrative sugar industry.

Years of neglect and nature took a toll on this 19th-century lighthouse.

The location of Puerto Rico's first water bottling plant, thanks to a collaboration between Mother Nature and technology.

This former sugarcane transport tunnel is now a portal to a lovely beach.

This bleach-white graveyard overlooking the ocean is accessed through a tunnel.

The world's second largest single-dish radio telescope, once used to contact E.T.

This chapel on a cliff is renowned for its healing powers and was founded on the site of a supposed divine intervention.

This demolished goliath of American industry once loomed over the depressed town it once made thrive.

One of the first commercially successful anthracite-powered iron-making operations in North America.

This Philly landmark has a giant butterfly pavilion, three floors of insect displays, and withstood a $40,000 heist.

Buildings that dynamite couldn't bring down were abandoned because of bathroom issues.

Hellam Township is reputed to feature seven remote gates, which when passed through in the right order, open an entrance to hell.

Constrained by time and topography, this civil engineering landmark was constructed 100 feet above Starrucca Creek over the course of a single year.

A piece of crumbling highway infrastructure was repurposed as a high-tech facility for testing racecars.

An underground blaze turned this village into a ghost town.

The lasting remains of the region's once booming cement industry.

This national landmark is also home to a massive treehouse.

An abandoned incline railway site is now a green space with six acres of trails connecting hilltop neighborhoods.

An industrial waste tourist attraction.

A beloved horticultural oasis on the grounds of a wealthy family's former estate.

An abandoned railroad tunnel transformed into a creepy path for hikers and cyclists.

The first of Lancaster's "rural cemeteries."

Narrow gorges and deep pools created by long-ago glaciers.